Ivermectin performance against equine strongylids: Efficacy, egg reappearance periods, and fecal egg counting method comparison.
Abstract: Equine strongylids are ubiquitous and can cause severe health issues. Anthelmintic resistance is widely common in cyathostomin parasites, and recent studies have documented increasing incidence of resistance to the macrocyclic lactone drug class. Several European countries have implemented prescription-only restrictions of anthelmintic usage by law to reduce anthelmintic treatment intensity and decrease the selection pressure for drug resistance. However, long term outcomes of this approach have not been thoroughly evaluated. The aim was to determine ivermectin efficacy in strongylids by means of faecal egg count reduction test (FECRT) and egg reappearance period (ERP) in Danish horses, compare two fecal egg count techniques, and determine the prevalence of Strongylus vulgaris in the study population. Herds and horses were selected by four collaborating veterinary practices. A total of 299 horses from 30 herds were enrolled in the study. Fecal egg counts (FEC) were determined using a concentration McMaster technique as well as an artificial intelligence-based automated fecal egg counting system. All horses with FEC > 0 eggs per gram of feces (EPG) were treated with ivermectin. Ivermectin efficacy and ERP were determined following current guidelines. Coproculture and PCR were employed for detection of S. vulgaris. FECRTs based on McMaster egg counts suggested inconclusive ivermectin efficacy in two equine operations and full efficacy in all remaining populations, whereas the automated system suggested ivermectin resistance in 6 operations and inconclusive results in another 8. Ivermectin ERP was determined to be at least 8 weeks with both methods in all cases. The prevalence of S. vulgaris was 2.7 % and 5.7 % with coproculture and PCR, respectively, and all samples were negative with both methods at 8 and 24 weeks post treatment. In general, ivermectin efficacy was high, although some results suggest that reduced efficacy could be occurring, which warrants further monitoring going forward. The ERP estimates all exceeded 8 weeks, which suggests that no reduction has occurred. The two egg counting techniques were in general agreement, but the automated system detected more positives at low egg count levels, which led to lower efficacy estimates in some populations. Strongylus vulgaris remains enzootic in Danish horses.
Copyright © 2025 The Authors. Published by Elsevier B.V. All rights reserved.
Publication Date: 2025-04-09 PubMed ID: 40215581DOI: 10.1016/j.vetpar.2025.110465Google Scholar: Lookup
The Equine Research Bank provides access to a large database of publicly available scientific literature. Inclusion in the Research Bank does not imply endorsement of study methods or findings by Mad Barn.
- Journal Article
- Comparative Study
Summary
This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.
The research focuses on assessing the efficacy of the drug Ivermectin against equine strongylids which are common parasites in horses. The study also compares two different methods of counting fecal egg counts, and investigates the prevalence of a specific parasite, Strongylus vulgaris, in Danish horses.
Overview of the Study
- The research concerns the performance of the drug Ivermectin against equine strongylids, parasites widespread in horses and responsible for severe health problems. The study arises from emerging resistance to common anthelmintic drugs, particularly in cyathostomin parasites.
- The study includes 299 horses from 30 different herds, selected by four collaborating veterinary practices in Denmark. The aim is to examine the efficacy of Ivermectin, the egg reappearance period (ERP), and to compare the techniques of fecal egg count.
Ivermectin Efficacy and ERP
- All horses with detectable eggs per gram of feces were treated with Ivermectin. The research used a faecal egg count reduction test (FECRT) to determine Ivermectin efficiency and evaluate the ERP.
- The ERP of Ivermectin was found to be at least 8 weeks in all instances, indicating no occurence of reduction. Also, the prevalence of Strongylus vulgaris, a parasite species, was around 2.7%-5.7% in the horse populations.
- Results overall showed that Ivermectin still had high efficiency. However, some isolated cases suggest potential reduced efficacy, necessitating further, future monitoring.
Comparison of Fecal Egg Count Techniques
- The research employed two distinct methods to count fecal eggs: a concentration McMaster technique and an artificial-intelligence-based automated system.
- The McMaster egg counts indicated inconclusive Ivermectin efficacy in two horse operations and full effectiveness in the rest, whereas the automated system suggested Ivermectin resistance in six operations and returned inconclusive findings for eight others.
- The two egg counting techniques generally gave concordant results. However, the automated system detected lower egg counts leading to lower efficacy estimates in certain populations.
Prevalence of Strongylus vulgaris
- The study determined the presence of Strongylus vulgaris in the horse population using coproculture and PCR testing.
- The prevalence of this parasite was found to be 2.7% and 5.7% with coproculture and PCR tests respectively. Additionally, all samples showed negative results at 8 and 24 weeks after treatment.
- Despite the high overall Ivermectin efficacy, the persistence of Strongylus vulgaris in Danish horses was confirmed.
Cite This Article
APA
Nielsen MK, Bartholdy ID, Kristensen KS, Borrye JC, Meilvang KSS, Rendtorff CEK, Hjortflod MD, Fuglbjerg V, Karlsson M, Petersen HH, Toft K, Thamsborg SM, Pihl TH.
(2025).
Ivermectin performance against equine strongylids: Efficacy, egg reappearance periods, and fecal egg counting method comparison.
Vet Parasitol, 336, 110465.
https://doi.org/10.1016/j.vetpar.2025.110465 Publication
Researcher Affiliations
- Department of Animal and Veterinary Sciences, Aarhus University, Tjele, Denmark. Electronic address: martin.nielsen@anivet.au.dk.
- Vestergade Dyreklinik, Vestergade 117, Silkeborg, Denmark.
- Distriktsveterinärerna Flyinge, Roslövsvägen 15, Flyinge, Sweden.
- Novo Nordisk, Girostrøget 1, Høje Taastrup, Denmark.
- Dyrlægegårdens Dyreklinik, Maribovej 90, Sakskøbing, Denmark.
- Mørkøv Dyreklinik, Holbækvej 173, Mørkøv, Denmark.
- Dyrlægegården Hundested, Nørregade 75, Hundested, Denmark.
- Højelse Hesteklinik, Ejbyvej 12C, Lille Skensved, Denmark.
- Ikeyvet, Jagtvej 5, Fredensborg, Denmark.
- Naturplejedyrlægen, Blommeskoven 1A, Gørlev, Denmark.
- Department of Veterinary Clinical Sciences, University of Copenhagen, Copenhagen, Denmark.
- Department of Veterinary and Animal Sciences, University of Copenhagen, Copenhagen, Denmark.
- Department of Veterinary Clinical Sciences, University of Copenhagen, Copenhagen, Denmark.
MeSH Terms
- Animals
- Horses
- Ivermectin / therapeutic use
- Ivermectin / pharmacology
- Parasite Egg Count / veterinary
- Parasite Egg Count / methods
- Feces / parasitology
- Strongyle Infections, Equine / drug therapy
- Strongyle Infections, Equine / parasitology
- Strongyle Infections, Equine / epidemiology
- Horse Diseases / drug therapy
- Horse Diseases / parasitology
- Strongylus / drug effects
- Female
- Denmark / epidemiology
- Anthelmintics / therapeutic use
- Anthelmintics / pharmacology
- Male
Conflict of Interest Statement
Declaration of Competing Interest Martin Nielsen is a shareholder of Parasight, Inc., the manufacturer of the automated fecal egg counting system utilized in this study. The remaining authors declare no conflicts of interest.
Citations
This article has been cited 0 times.Use Nutrition Calculator
Check if your horse's diet meets their nutrition requirements with our easy-to-use tool Check your horse's diet with our easy-to-use tool
Talk to a Nutritionist
Discuss your horse's feeding plan with our experts over a free phone consultation Discuss your horse's diet over a phone consultation
Submit Diet Evaluation
Get a customized feeding plan for your horse formulated by our equine nutritionists Get a custom feeding plan formulated by our nutritionists
